The Time Remapping shortcut is Ctrl(Cmd)+Alt(opt)+T. This puts keyframes on the clip that you can move to change the speed. This is great for speed ramps and other variable time effects. You can also Ctrl(Cmd)+Alt(opt)+click on the keyframes to create hold keyframes, which will freeze that frame.

The Time Reverse shortcut is Ctrl(Cmd)+Alt(opt)+R. This does what the name suggests, it reverses the layer and you'll see a red line on the bottom of the clip.

To open the Time Stretch panel just click on the {} icon in the bottom left of the timeline panel. To Dave's point, it can make the footage look jerky if you enter a percentage that isn't evenly divisible by the frame rate. Using values like 200% or 50% are always safe (unless you are using an odd number framerate like 15 or something).

Whenever you Time Stretch above 100% for slow motion you can turn on Frame Blending (the double filmstrip icon in the timeline panel) to have AE create new frames in between existing frames instead of repeating frames, which can look jerky. One click and you'll see a jagged backslash, which is half quality. Click again and it turns to a forward slash, which is full quality. The results can vary, depending on the amount and predictability of the motion in the clip, among other factors.

To see the effect when you RAM preview you have to also enable Frame Blending for the comp in the top of the timeline panel AND set the comp resolution to Full.